Methanol is one of the major chemical raw materials, ranking third in volume behind ammonia and ethylene. Worldwide demand for methanol as a chemical raw material continues to rise especially in view of its increasingly important role (along with dimethyl ether) as a source of olefins such as ethylene and propylene and as an alternative energy source, for example, as a motor fuel additive or in the conversion of methanol to gasoline.
Methanol (as well as dimethyl ether) can be produced via the catalytic conversion of a gaseous feedstock comprising hydrogen, carbon monoxide and carbon dioxide. Such a gaseous mixture is commonly referred to as synthesis gas or “syngas”.
Methanol is typically produced from the catalytic reaction of syngas in a methanol synthesis reactor in the presence of a heterogeneous catalyst. For example, in one synthesis process, methanol is produced using a copper/zinc catalyst in a water-cooled tubular methanol reactor. In methanol production, syngas undergoes three reactions, only two of which are independent. These reactions are:2H2+COCH3OH  (1)3H2+CO2CH3OH+H2O  (2)H2O+COH2+CO2  (3)
As can be seen from Reactions #2 and #3, CO2 can participate in methanol synthesis. Nevertheless, it is desirable to minimize the amount of CO2 in the syngas for several reasons. In the first place, a low CO2 content in the syngas results in a more reactive mixture for methanol synthesis provided the CO2 content is at least about 2%. Furthermore, less CO2 results in lower consumption of hydrogen and lower production of water. Lower water production is useful in applications where some relative small amounts of water can be present in the methanol product such as, for example, in connection with a methanol to olefins (MTO) process. Production of methanol with low water content thus eliminates the need to distill water from the syngas product methanol.
The syngas stoichiometry for methanol synthesis from syngas is generally described by the following relationship known as the “Stoichiometric Number” or SN.SN=(H2−CO2)/(CO+CO2)  (4)
The value of SN theoretically required for methanol synthesis is 2.0. However, for commercial production of methanol from syngas, it is desirable that the value for SN range from about 1.95 to 2.15. Dimethyl ether (DME) may also be produced from syngas using chemistry similar to that used for methanol synthesis.
Autothermal reforming (ATR) involves the addition of air or oxygen with relatively smaller proportions of steam to a hydrocarbon feedstock. Reaction of hydrocarbon with oxygen proceeds according to the following general reaction schemes:CnHm+(n/2)O2nCO+(m/2)H2  (5)andCnHm+(n+m/4)O2nCO2+m/2H2O  (6)
When methane is the hydrocarbon undergoing oxidative reforming, these reactions become:CH4+½O2CO+2H2  (7)andCH4+2O2CO2+2H2O  (8)
Autothermal reforming employs both steam reforming and oxidative reforming of the hydrocarbon feed. The exothermic oxidation of the feedstock hydrocarbons generates sufficient heat to drive the endothermic steam reforming reaction over the catalyst bed. The ATR procedure is thus run at relatively high temperatures and pressures with a relatively low steam to carbon ratio. The CO2 content of the syngas from ATR processes, however, is fairly low, as is desirable for methanol synthesis.
Another known reforming process involves primarily partial oxidation of a hydrocarbon feed with an oxygen-containing gas. Catalytic partial oxidation reforming procedures are known; for purposes of this invention, partial oxidation reforming takes place in the absence of a catalyst. Due to the absence of a catalyst, partial oxidation (POX) reforming can operate at very high temperatures with little or no steam addition to the feedstock. Higher pressures than are used in ATR operations can be employed in POX reforming. However, the syngas composition resulting from POX reforming is generally deficient in hydrogen for methanol synthesis, resulting in SN and H2:CO numbers below 2. On the other hand, the CO2 content of the resulting syngas is generally very low which is below the optimum value for methanol synthesis.
It is known in the prior art to utilize various combinations of reforming operation types and procedures in the preparation of syngas mixtures which can be converted, for instance, into oxygenates. For example, Texaco's U.S. Pat. No. 5,496,859 discloses a method for the production of a “stoichiometric ratioed syngas”. The method comprises partially oxidizing a gaseous feedstock containing substantial amounts of methane in a gasifier to produce a hot synthesis gas stream that is passed in indirect heat exchange through a steam reforming catalytic reactor. A portion of the steam reforming reaction products are mixed with the cooled gasifier synthesis gas stream exiting the steam reforming catalytic reactor to form a stoichiometric ratioed synthesis gas. The stoichiometric ratioed synthesis gas stream can then be passed into a methanol synthesis unit at substantially the specifications for optimal methanol production with little or no external compression. The stoichiometric ratio, SN, in the syngas produced is said to range from 1.9 to 2.1. Syngas having an SN of 1.9 and an H2/CO ratio of 2.52 are exemplified in the '859 patent. The exemplified syngas has an excessively high CO2 content of 5.3% (on a water-free basis).
Haldor Topsoe's U.S. Pat. No. 6,224,789 and related publication [Aasberg-Petersen et al.; Applied Catalysis, A: General (2001), 221 (1-2), pp. 379-387] both disclose an arrangement similar to that of the Texaco '859 patent wherein effluent gas from an ATR unit circulates around and supplies heat to the HTCR (a heat exchanger version of a steam reforming reactor), but does not undergo chemical reaction there. The exemplified Haldor Topsoe process provides a syngas with a SN stoichiometric ratio of 3.66 and a H2/CO ratio of 3.25.
Shell's U.S. Published Application No. 2004/0241086 discloses a process for the preparation of syngas from a gaseous hydrocarbon feedstock by (a) partial oxidation of a part of the feedstock and (b) steam reforming of another part of the feedstock. The mixture obtained in step (b) may be directly combined with the product gas as obtained in step (a). No description of feedstock or syngas composition characterized by component concentration is given.
ICI's U.S. Pat. No. 5,252,609 discloses a syngas production process involving the steam reforming and oxygen-blown reforming treatment of two separate hydrocarbon feedstock streams. Such a process comprises (a) steam reforming a first stream of desulfurized hydrocarbon feedstock, optionally, followed by secondary reforming using an oxygen-containing gas and then cooling, (b) steam reforming a second stream of the feedstock, preferably adding a hydrogen-containing gas, and then subjecting the product to partial oxidation with an oxygen-containing gas and then cooling, and (c) mixing the two cooled product streams.
Shell's WO 04/092062 and WO 04/092063 both disclose other syngas production processes involving a combination of different types of reforming operations. Such processes comprise (a) partial oxidation of a carbonaceous feedstock thereby obtaining a first gaseous mixture of hydrogen and carbon monoxide, (b) steam reforming a carbonaceous feedstock, wherein the steam to carbon molar ratio is below 1, to obtain as a steam reforming product, (c) feeding the steam reforming product to the upper end of the partial oxidation reactor to obtain a mixture of the effluent of step (a) and the steam reformer product, and (d) providing the required heat for the steam reforming reaction in step (b) by convective heat exchange between the mixture obtained in step (c) and the steam reformer reactor tubes, thereby obtaining a hydrogen and carbon monoxide containing gas having a reduced temperature.
Praxair/Standard Oil's U.S. Pat. No. 6,402,988 discloses the following: “An exothermic reaction and an endothermic reaction are thermally combined in a reactor having at least one oxygen selective ion transport membrane that provides the exothermic reaction with oxygen from an oxygen-containing gas such as air. The thermal requirements of the endothermic reaction are satisfied by the exothermic reaction. Dependent on the reactor design employed, the exothermic and endothermic reactions may be gaseously combined”. The exothermic reaction is partial oxidation; the endothermic reaction is steam reforming.
Davy McKee's WO 93/15999 shows an arrangement, with steam reforming and partial oxidation (POX) units in parallel. The effluents are combined and in this arrangement are sent to a secondary reforming zone for further reduction in product methane content.
U.S. Pat. No. 5,310,506 discloses the production of synthesis gas for methanol synthesis and reacting the synthesis gas in the presence of a catalyst to produce a high-methanol product stream. Water at 1.2 to 2.0 molecules water per carbon atom in the feed gas, oxygen at 0.4 to 0.8 molecules oxygen per carbon atom in the feed gas, a high-hydrogen gas which contains free hydrocarbon at 0.2 to 0.5 molecules hydrogen per carbon atom in the feed gas, and a feed gas containing methane are fed into an ATR. The high-hydrogen gas comes at least in part from a plant other than from the instant methanol process. The reactor is operated at a pressure of 10 to 100 bars (1 to 10 MPa) and the raw synthesis gas is withdrawn at a temperature in the range of from 850° C. to 1100° C. The raw synthesis gas contains methane not in excess of 3 mol %, without removal of carbon dioxide producing a synthesis gas with a SN of from 1.97 to 2.2.
Although a variety of autothermal reforming operation systems have been proposed and methods of adjusting the SN have been proposed, additional and further efficient systems are sought that incorporate external hydrogen streams in order to adjust the SN.
By adding a hydrogen stream to a reforming reactor, the temperature of the reforming reaction can be reduced, but the conversion rate is maintained and the SN is adjusted to the appropriate level. Another option is to add a hydrogen stream after the reforming reactor, which allows for greater capacity in the reformer and adjusts the SN to the appropriate level. Another option is to add hydrogen to the reforming reactor and to the synthesis gas withdrawn from the reforming reactor in order to adjust the SN to the appropriate level.